Improving the NOx decomposition and storage activity through co-incorporating ammonium and copper ions into Mg/Al hydrotalcites

Abstract

We report a 40% and 70% improvement of NO_x decomposition and storage rate based on synthetic Mg/Al/Cu/NH₄⁺ hydrotalcites (HT), compared with Mg/Al/NH₃ and Mg/Al/Cu HTs. TGA and DTG show that ammonium has been released from HT below 160 °C. A combined characterization through WAXS (Wide-Angle X-ray Scattering), SEM, sXAS (soft X-ray Absorption Spectroscopy), CHNS elemental analysis and N₂ absorption and desorption reveals the structural and physical properties of this co-incorporated system. While Mg/Al/Cu/NH₄⁺ HT retains the typical structure of hydrotalcite-like compounds, its crystallinity weakened. WAXS shows that the layer–layer spacing of Mg/Al/Cu and Mg/Al/Cu/NH₄⁺ HTs decreases due to the Jahn–Teller effect. The particle size of Mg/Al/Cu/NH₄⁺ decreases about 10 nm compared with that of Mg/Al/Cu HT. sXAS and CHNS elemental analysis reveal that ammonium was successfully introduced into Mg/Al/Cu/NH₄⁺ HT by analyzing the N-K edge and CHN content, but not into HT without Cu²⁺, indicating the importance of the co-incorporation of ammonium and Cu²⁺. Additionally, the test of NO_x decomposition and storage activity revealed that the introduction of ammonium into the system should be responsible for the performance improvement.